There have been known a read-only optical disc (e.g., compact disc, optical video disc, etc.) in which data based on various kinds of information signals cannot be erased and rerecorded at all, a write-once type optical disc in which data can be recorded but cannot be erased, and an optical disc in which data based on various kinds of information signals can be erased and rerecorded. For example, a magnetooptic disc has been known as such a type of optical disc. In a magnetooptic disc, data is recorded by the magnetization direction in a manner similar to the perpendicular magnetic recording system. Namely, in a magnetooptic disc, a perpendicular magnetization film in which the magnetization direction is oriented perpendicularly to the disc surface is used as a recording medium. This recording medium has a certain coercive force at an ordinary temperature, whereby the magnetization direction does not change, and in an initial state before data is recorded or in a state in which the recorded data was erased, the magnetization direction is oriented in a constant direction. When a recording beam, e.g., a laser beam, is irradiated onto a recording medium, a temperature of the beam irradiated portion rapidly increases. When the temperature has reached a Curie temperature or a compensation temperature, the coercive force decreases. Therefore, a weak magnetic field is applied from the outside by an external magnetic field generating means. In this state, by irradiating a recording beam, e.g., applying a laser beam onto a recording medium of a disc, the temperature of the beam irradiated portion promptly increases and the magnetization direction of the recording medium is reversed in accordance with the direction of the magnetic field generated from the external magnetic field generating means. Thus, data is written. In the case of reading data, a reproducing beam, e.g., a laser beam having a power which is sufficiently smaller than the irradiation power when data is written, is irradiated onto a disc, and a rotational angle of a rectilinear plane of polarization of the reflected light is detected.
On the other hand, in the case of erasing the data written in the disc, the direction of the magnetic field which is generated from the external magnetic field generating means is held constant and at the same time, by irradiating an erasing beam, e.g., applying a laser beam onto a desired portion of a recording medium of the disc, the magnetization direction of the recording medium is oriented to a constant direction in accordance with the direction of the magnetic field generated from the external magnetic field generating means on the basis of a principle similar to that when data is written. In this manner, data is erased.
Hitherto, as shown in FIG. 1, a guide groove 31 for tracking is formed in such an erasable and rerecordable optical disc 30. A laser beam is emitted from a laser light source of an optical head (not shown) which can be freely moved in the radial direction of an optical disc. A tracking servo is applied to an ojective lens drive apparatus (not shown) of the optical head so that the emitted laser beam can trace along the guide groove 31, so that data is recorded along the guide groove 31. As shown in FIG. 2, an area of the disc 30 is divided into, e.g., 32 sectors per rotation. Data is read out or written every sector.
In the case of an erasable and rerecordable optical disc, as mentioned above, a laser beam is used to read out and write data. A laser diode is used as a laser beam emitting source. The optimum value of the power of the laser beam differs in each of the reading, writing, and erasing modes. Namely, as mentioned above, in the case of writing data, in order to reverse the magnetization direction of a recording medium of an optical disc, a laser beam from a laser diode is irradiated onto the recording medium and a temperature of the recording medium needs to be raised to a temperature near the Curie temperature or the compensation temperature. Therefore, the largest power is necessary in the writing mode. If a sufficiently large power is not obtained in the writing mode, the temperature of the recording medium in the portion to which a laser beam was irradiated does not reach the Curie temperature, so that the magnetization direction cannot be reversed. Therefore, data is not sufficiently written. On the other hand, if the power is too large in the reading mode, data is further written by a reading laser beam onto the data which has already been written into a disc, so that the written data may be broken. As shown in FIG. 3, the power of a laser diode largely changes due to a change in temperature and also largely varies in dependence on an elapse of time.
Therefore, in the case of using a laser diode as a laser light source of an apparatus for recording and reproducing data into/from an optical disc, it is necessary to use a servo circuit adapted to keep the laser diode power of the optimum value. Such a servo circuit is called an automatic power control (APC) circuit.
FIG. 4 shows an example of an APC circuit for use in a conventional magnetooptic disc recording and reproducing apparatus. In FIG. 4, a laser diode 21 is the light source. A laser beam from the laser diode 21 is irradiated onto a magnetooptic disc. Light emission power of the laser diode 21 is detected by a monitoring photo diode 22 disposed near the laser diode 21.
In general, the light fluxes corresponding to an intensity of an emitted laser beam (in this case, the light fluxes are referred to as monitor lights) are emitted from both the edge surface of the laser diode that emits the laser beam onto the disc and also from the edge surface on the opposite side thereof. These monitor lights are detected by the photo diode 22. Namely, the monitor lights according to the light emission power of the laser diode 21 are received by the photo diode 22 and a current corresponding to the received monitor lights flows through the photo diode 22.
A detection current of the photo diode 22 is converted into a voltage value by a current-voltage (I-V) converting circuit 23. An output of the I-V converting circuit 23 is supplied to a comparator 24. Target voltages Vr.sub.11, Vr.sub.12, and Vr.sub.13 are selectively supplied to the comparator 24 through a switch 25. Namely, when terminals 25A and 25D of the switch 25 are connected, the target voltage Vr.sub.11 for the writing mode is supplied from a voltage source 26A to the comparator 24. When terminals 25B and 25D are connected, the target voltage Vr.sub.12 for the erasing mode is supplied from a voltage source 26B to the comparator 24. When terminals 25C and 25D are connected, the target voltage Vr.sub.13 for the reading mode is supplied from a voltage source 26C to the comparator 24. In this manner, each target voltage is selectively supplied to the comparator 24 by the switch 25 whose switching operation is controlled by control means (not shown) in accordance with each of the writing, erasing, and reading modes.
The output of the comparator 24 is supplied to a current limiter 28 through a low pass filter 27. The current limiter 28 is provided to prevent an overcurrent from flowing through and breaking the laser diode 21. An output of the current limiter 28 is supplied to a drive circuit 29 and an output of the drive circuit 29 is supplied to the laser diode 21. A laser beam corresponding in intensity to the output of the drive circuit 29 is emitted from the laser diode 21 and irradiated onto the magnetooptic disc. On the other hand, in the writing mode, a modulation signal based on an information signal to be recorded is supplied from a terminal 32 to the drive circuit 29. Thus, a laser beam is modulated by the modulation signal and the switch 25 is switched by control means (not shown) and the laser beam having a predetermined power in the writing mode is generated from the laser diode 21.
As mentioned above, the output of the laser diode 21 is detected by the photo diode 22. The detection output of the photo diode 22 is compared with the target value in each of the writing, erasing, and reading modes by the comparator 24. The light emission output of the laser diode 21 is controlled so as to become constant at the target value in each of the writing, erasing, and reading modes in accordance with the output of the comparator 24.
However, it takes an appreciable amount of time until the APC circuit becomes stable. Namely, for example, when the operating mode was changed from the reading mode to the writing mode, the target voltage is switched from Vr.sub.13 to Vr.sub.11 and the light emission power of the laser diode 21 is increased. However, the power of the laser diode 21 does not immediately rise to the target value in the writing mode from the target value in the reading mode and is not stable until it reaches the target value. Therefore, as shown in FIG. 2, a conventional optical disc is provided with a gap G between sectors. No effective data is recorded in the gap G. After the operation of the APC circuit has stabilized, the writing, reading, or erasing operation is performed.
Therefore, in a conventional optical disc, since the gap G in which effective data is not recorded until the APC circuit becomes stable is formed between sectors, a recording area of the recording medium of the disc cannot be effectively used and recording density cannot be increased. Further, as mentioned above, the guide groove 31 for tracking is formed in a conventional optical disc. Such a guide groove 31 causes a problem in that noise from the guide groove 31 is picked up, or a carrier-to-noise (C/N) ratio deteriorates and the reliability of data also deteriorates. To avoid such a problem, a method whereby a tracking servo is applied without forming the guide groove 31 is considered. Namely, for example, as disclosed in U.S. Pat. No. 4,443,870 and the like, a plurality of servo areas are provided in a disc and servo pits for tracking are formed in the servo areas. The tracking servo is applied in the servo area interval on the basis of the tracking error information which is obtained from the servo pits.
In the case where a tracking error is detected from the servo pits in the servo area and a tracking is controlled as described above, response speed of the APC circuit becomes a problem. Namely, in the case where tracking control was performed by the foregoing method, the servo area is certainly set into the reading mode in order to read out the servo pits. Since the servo area certainly enters the reading mode, in the case of writing data into a disc, it is necessary to set the power of the laser diode to the optimum value in the reading mode in the servo area and the power of the laser diode needs to be immediately set to the optimum value in the writing mode in a recording area. Namely, in FIG. 4, the terminals 25C and 25D are connected in the servo area and the terminals 25A and 25D are connected in the recording area. However, since it takes a predetermined time until the operation of the APC circuit becomes stable, when a laser beam moves from the servo area to the recording area, the power of the laser diode does not promptly become the optimum value.
Therefore, it has been proposed that a gap such as the gap G provided between sectors as shown in FIG. 2 be formed between the servo area and the recording area. However, the formation of gaps results in a decrease in recording density of the disc in a manner similar to the case of the optical disc in FIG. 2, so that the recording area for effective data is hardly obtained. On the other hand, it has also been proposed to omit the low pass filter 27 in order to increase the operating speed of the APC circuit. However, according to this proposal, the operation of the APC circuit does not become stable.